1. Field of the Invention
The present invention generally relates to optical transmission systems and networks, and more particularly to methods and systems for reducing four-wave mixing (FWM) in optical transmission networks.
2. Technical Background
FWM is a non-linear effect exhibited by optical waveguide fibers when multiple wavelengths (frequencies) are simultaneously transmitted through the fiber as, for example, in wavelength division multiplexed (WDM) systems. In particular, when at least two signals at different frequencies are transmitted through a fiber, the two signals will interfere and generate FWM cross-talk product of different wavelengths. See, for example, the dashed lines in FIG. 1, which represent transmitted signals and the resultant FWM products. The more optical signals that are transmitted through the fiber, the more FWM cross-talk products are generated, since there are more signals to interfere with one another. When transmitting through the fiber over long distances, the fiber is typically divided into spans, with in-line optical amplifiers positioned between the spans. A typical span is, for example, 80 km in length. Each time the transmitted signals are amplified by one of the in-line optical amplifiers, the FWM products are amplified, and additional FWM products are generated due to the interference of the FWM products with each of the modulated optical signals and the interference of the FWM products with each other.
The strength of the FWM products depends on the power levels at the original starting frequencies, the fiber dispersion, and the channel spacing. The use of optical amplifiers to achieve longer unrepeated lengths in optical fiber transmission systems has resulted in higher power levels and thus stronger FWM products. In addition, when dispersion-shifted fibers are used, FWM is enhanced as a result of the reduction of the phase mismatch naturally provided by fiber dispersion.
The International Telecommunication Union""s (ITU) standards for dense wavelength division multiplexed systems further exacerbate the problem. Specifically, the ITU has specified that WDM systems should have equal frequency spacing between channels within a certain tolerance, e.g., a 200 GHz channel spacing with a tolerance of xc2x140 GHz about the center frequency of each channel (the ITU grid). Exact equal spacing results in many of the FWM products coinciding (overlapping) with the channel frequencies. Crosstalk is thus maximized when the ITU standards are achieved, i.e., when the center frequency for each signal is at the ITU standard.
As a result of these considerations, FWM is today one of the limiting non-linear processes in optical fiber transmission systems. A number of proposals which employ unequal spacing between signal channels to address this problem have appeared in the literature. See J-S. Lee and D-H. Lee, OFC Proceedings, FC5, 393 (1998); Y. Hamazumi, M. Koga, and K. Sato, IEEE Photonics Technol. Lett., 8, 718 (1996); F. Forghieri, R. W. Tkach, A. R. Chraplyvy, and D. Marcuse, IEEE Photonics Technol. Lett., 6, 754 (1994); and F. Forghieri, R. W. Tkach, and A. R. Chraplyvy, J. Lightwave Technol., 13, 889 (1995).
These schemes for locating signal channels suffer from a number of deficiencies. First, they rely on extensive mathematical calculations to determine the optimum position of the channels, i.e., the position where minimum overlap with the FWM products is achieved. Although these algorithms can determine the optimum position where the FWM penalties are at their minimum, the resulting channel positions are complicated to implement in practice, since they involve a set of channel frequencies, which have complex relationships to one another.
Second, the schemes leave numerous unused slots between channels, which results in a significant expansion of the transmission bandwidth, a clearly undesirable result, since the overall goal of WDM is to place as many channels as possible into a given bandwidth.
Finally, the schemes generally do not result in channel locations which meet the requirements of the ITU grid. Thus, although they can reduce FWM, such reduction may be at the expense of standardization of WDM technology on a worldwide basis.
Other approaches to reduce FWM products include dithering of the source laser frequencies. See K. Inoue, xe2x80x9cReduction of Fiber Four-wave Mixing Influence Using Frequency Modulation in Multi-channel IM/DD Transmission,xe2x80x9d IEEE Photon. Technol. Lett., Vol. 4, No. 11, pp. 1301-1304 (1992). Frequency dithering requires frequency modulation of all individual WDM laser sources, which may result in substantial and residual amplitude modulation that degrades system performance. Thus, frequency dithering has not been used in practice.
An aspect of the invention is to provide a system for reducing FWM without the above-described detrimental effects. Specifically, it is an aspect of the present invention to provide a system for reducing FWM without adjusting the channel frequencies. To achieve these and other aspects and advantages, a method is provided that comprises modulating the phase of the optical signals propagating through a long fiber waveguide at a modulation frequency that causes destructive interference of FWM products that are otherwise generated along the length of the long fiber waveguide. This method may be implemented in a transmitter for an optical transmission network by providing a phase modulator between the multiplexer and an optical boost amplifier to simultaneously modulate the phase of all the optical signals that are transmitted through the long fiber waveguide.
Additional features and advantages of the invention will be set forth in the detailed description which follows and will be apparent to those skilled in the art from the description or recognized by practicing the invention as described in the description which follows together with the claims and appended drawings.